Hello DCR944
Since regular point-to-point tickets as well as Saver Day Passes offer the freedom to choose any connection, there is no guarantee for a seat. Here in Switzerland, tickets do not come with a seat allocation. We use the train system like you probably use the buses in your city. In the worst case you have to be standing a few stations.
What concerns your trip, avoid using EC trains. Use the regular IR trains from Visp to Geneva Airport, there will certainly be seats left on these trains. And, plan enough buffer to arrive at Geneva Airport in good time.
Kind regards,
Roland